1. Field of the Invention
The present invention relates to a vibratory flowmeter and method, and more particularly, to a corrosion-resistant vibratory flowmeter and method.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things.
Vibratory flowmeters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow tubes that are vibrated in order to measure a fluid. Such flow tubes are commonly formed of metal in order to possess good vibrational characteristics and high strength, such as for high-pressure flow fluid applications.
However, vibratory flowmeters are often used for handling corrosive and/or chemically reactive materials. Metal flow tubes can be damaged by corrosive or reactive flow fluids. In addition, metal flow tubes may leach material from the flow fluid and/or the flow fluid may leach material from the flow tubes. Further, the coating or coatings may prevent erosion by providing a very hard surface. Moreover, the coatings may be slippery or present low coefficients of friction to a flow.
FIG. 1 is a cross-section of a prior art flow tube that includes an additional lining formed inside the interior of the tube. Such a prior art lining typically comprises TEFLON, TEFZEL, or other plastics, for example, and provides a corrosion-resistant and chemically inert lining. As a result, the flow fluid does not contact the metal flow tube.
However, the prior art has drawbacks. The prior art lining comprises a separate component that must be deposited, bonded, or adhered to the inner surface of the prior art flow tube. Unfortunately, the prior art lining may delaminate from the interior of the flow tube. The delamination may result in leakage behind the prior art lining, corrosion, and flow obstruction. In extreme cases, the delamination may result in breakage of the lining and broken pieces of the lining may join the flow fluid and affect downstream filters, valves, pumps, or other mechanisms. Further, free pieces of the lining may affect downstream mixtures, chemical processes, equipment, or measurements.
The prior art lining typically is much softer than the metal of the prior art flow tube. Therefore, the prior art lining must be significantly thick in order to provide an adequate barrier and prevent flow fluid contact with the underlying flow tube. Also, the prior art lining must be significantly thick in order to not collapse or be adversely structurally affected by vibration. Unfortunately, the thickness of the prior art lining will affect the vibrational characteristics of a prior art flowmeter assembly. More drive power may be required. Sensitivity may be decreased. The resonant frequency of the flowmeter will be changed.